So, what are you asking of us ? Reading your narrative, it seems clear the contravention occurred due to you not displaying a permit for April. I see, though, that there is a photo of a permit for April, which I assume you obtained following service of the PCN.
Did you display the permit for March, as the PCN is for 5th April, not too many days into April ? If so, I would submit informal representations asking for them to cancel the PCN on the basis of goodwill, you having been attending the leisure centre since JUly 2024, and took out a years subscription in November, but on this ocasion, it being early April, you had missed it now being April and had forgotten to take out a monthly permit. Include a copy of your March and April permits.
As far as I can see the PCN looks OK, but to be sure we need to see all sides of it.